For the production of crude triglyceride oils and fats, solvent extraction of oleaginous materials has been a widely practiced process for almost a century. If the oil or fat content of the oleaginous material is high, for instance above 20% by weight or more, the solvent extraction process may be preceded by a mechanical expelling step but to attain low residual oil levels, extraction is mandatory. Accordingly, an oilseed crushing plant may provide the following treatments: seed purification, seed conditioning, seed flaking to open the oil-containing cells, pressing or expelling the oil from the flakes and producing a cake, extracting the cake with an organic solvent, such as, but not limited to n-hexane, to produce a miscella and a marc, evaporating the solvent from the miscella to produce crude oil and desolventising the marc to produce defatted meal.
More recently, an oleaginous material that formerly was not extracted with a solvent is now also treated by this process but for a different reason. Fishmeal is commonly produced from fatty fish and fish offal by a kind of wet rendering process. In this process, the fishy raw material is comminuted in a shredder and then cooked in water. Liquid is then drained away from the cooked minced material, which is then pressed to expel further liquids. The liquids are separated into an oily phase and so-called stick water, which contains some dissolved proteins and is therefore combined with the expeller cake to yield fishmeal after drying. To prevent the proteins in the fishmeal from denaturing, its temperature is maintained below 80° C. so that the meal is dried under vacuum. However, this fishmeal still contains some oil and for some meal applications, it has to be extracted with an organic solvent.
The reason for this extraction is not so much the isolation of the oil but the removal of the oil from the meal since the oil contains chlorinated hydrocarbons such as, but not limited to, dioxins and pesticide residues, which are concentrated in the oil since these residues are fat-soluble. In the food chain they can therefore accumulate to unacceptably high levels. Accordingly, removing the oil from fishmeal exhibiting such high levels will lower the content of chlorinated hydrocarbons in the meal to acceptable levels, allowing the meal to be used as a feed component in for instance the feed used in fish farming. Consequently, fishmeal may be extracted with an organic solvent yielding a marc that has to be desolventised in such a way that the fish protein does not denature.
Standard desolventisers, as commonly used in the crushing industry, employ a combination of indirect and direct heating with steam to evaporate the solvent. As disclosed in U.S. Pat. No. 4,622,760, the desolventisation proper, by direct action of a desolventising agent, particularly steam, is relieved by interposing before the desolventisation proper a pre-desolventisation with indirect heat transfer, particularly indirect steam heating, and thereby the energy input of the entire system is reduced considerably. Moreover, the drying and cooling of the now solvent-free residue, which follows the desolventisation, can take place in a single, combined stage, whereby further relief of the volume of the apparatus, as well as the energy input, is achieved.
Accordingly, the marc entering the apparatus is first of all heated indirectly. This causes part of the solvent to evaporate but, as long as the material being desolventised still contains some solvent, its temperature will not rise to above the boiling point of the solvent at the prevailing pressure. In a second stage, direct steam causes the remaining solvent to evaporate and the temperature of the material being desolventised to rise to close to the boiling point of water at the prevailing pressure. The treatment of the material being desolventised at this elevated temperature and moisture level is commonly referred to as toasting. In the case of soya bean meal, toasting is an important part of the desolventising process because during toasting, anti-nutritional factors such as, but not limited to anti-trypsin, are denatured as a result of which the nutritional value of the meal increases.
Because the direct steam condenses onto the meal, its moisture content increases, and this assists in the denaturation of the anti-nutritional factors. However, before the meal can be stored, its temperature must be lowered and its moisture content must be reduced to below 13% by weight. This is achieved by blowing air through the toasted material. This air evaporates the water and in doing so cools the material at the same time. However, this air may not be mixed with the solvent vapours in the desolventising compartments because that might lead to the formation of an explosive mixture. Accordingly, the drying and cooling treatments can be applied in a separate vessel. If these treatments are carried out in the same vessel, its top compartments are separated from its bottom compartments, and the material being treated is transported from the top compartments to the bottom compartments by a rotary valve that provides a sufficient barrier for gas to travel from one compartment to the other.
The toasting treatment described above not only eliminates the anti-nutritional factors as present in soya beans, it also causes the water-solubility of the proteins present in the meal to decrease. For some meal applications such as, but not limited to ruminant feed, this is an added advantage but it can also be a disadvantage. For example, the isolation of soya bean protein from soya bean meal demands that this protein is soluble in water and for this reason, toasted soya bean meal is not a suitable starting material for the production of soya isolates; instead, so-called white flakes are used as the starting material. These white flakes have been obtained by desolventising soya marc without toasting this material at the same time. For the nutritional quality of fishmeal used in aquaculture, a low degree of protein denaturation is also desirable, since it increases the feed to fish conversion factor.
U.S. Pat. No. 3,392,455 discloses a process for removing residual volatile substances from a particulate solid material and heat treating the solid material including the steps of determining the maximum temperature that the material should be subjected to during the processing of the material, contacting the material with vapours of the volatile substances, and placing the material in a pressure-tight chamber, placing an inert gas in the chamber, maintaining the temperature of the material in the chamber below the maximum temperature, and varying the time and pressure of processing to remove the solvent. Accordingly, a sub-atmospheric pressure is maintained in the chamber to lower the evaporation temperature of the solvent and thereby prevent the protein from denaturing. However, the process disclosed in U.S. Pat. No. 3,392,455 has the disadvantage that its rotary valves inherently introduce a substantial amount of air when transporting the material.
This disadvantage has been overcome by the apparatus and process disclosed in U.S. Pat. No. 3,367,034, for the continuous separation of solvents from finely-composed or powdery particles, such as pulverized meal, wherein the material is contacted with super heated continuously recycled solvent vapour, flash separating solvent from the material, then contacting the material with an inert gas, then contacting the material with a flow of cold air. Accordingly the apparatus disclosed in U.S. Pat. No. 3,367,034 includes several vapour seal valves, but since these valves are no longer vapour tight, if there is a marked pressure difference across the valves, the apparatus cannot safely operate at sub-atmospheric pressures.
Accordingly, there is still a need for a desolventising system that is capable of operating continuously, at sub-atmospheric pressure, with enhanced gas-impermeability at the feeding and/or discharging ends.